X-ray systems are often used to perform angiographic examinations. Examples of x-ray systems suitable for such examinations include digital fluoroscopy, analog fluoroscopy, spot imaging and planar tomography. In such systems, an x-ray source is disposed on one side of a patient and an x-ray detector is disposed on the other side. The x-ray detector converts x-rays which have passed through the patient into secondary carriers (i.e. visible light) which are converted to a video signal. Because blood is relatively transparent to x-rays, the patient is injected with a radiopaque dye which has relatively good x-ray absorption such that blood vessels are more conspicuous in a resultant image. Images of a circulatory system may then be obtained by subtracting a processed reference or basis image taken before injection of the dye from a processed image taken after injection of the dye.
One application of x-ray angiography is imaging blood flow in a patient's lower extremities. The radiopaque dye is introduced into an artery in the pelvic or lower abdomen area and flows with the blood down the patient's leg. The dye is then imaged as it flows down the leg. Such a procedure is often referred to as an angiographic femoral runoff. In a normal healthy patient with good circulation, the dye moves from the pelvic area to the toes fairly quickly, whereas in a patient with artery blockage the dye may take a significantly longer time to reach the toes or may not reach there altogether. Thus, by virtue of examining the distal most conspicuous portion of the artery or vessel injected with the dye, areas of possible artery blockage may be inferred.
During x-ray imaging, an intensity level of x-rays reaching the x-ray detector is monitored to assure that the overall intensity of x-rays received by the x-ray detector is satisfactory to produce an image of diagnostic quality. The intensity of the x-rays received by the x-ray detector is measured in terms of exposure. If it is determined that the x-ray detector is not receiving a satisfactory exposure to x-rays, a signal is sent to the x-ray source to increase either one, or both, of the number of x-rays produced and/or the energy of the x-rays.
One difficulty associated with producing angiographic and other x-ray images is that x-rays incident on the x-ray detector are either of an intensity level which is too high or too low thereby resulting in reduced image quality. Take, for example, a situation in which a patient is lying horizontally on a patient support having an x-ray source disposed below the patient which transmits x-rays to an x-ray detector positioned above the patient. An imaging region as seen by the x-ray detector will often include regions representative of gaps between the patient's anatomy as commonly exist, for example, between the patient's legs in an angiographic femoral runoff examination. Such gaps allow unattenuated x-rays to reach the x-ray detector. Unfortunately, the high exposure associated with unattenuated x-rays cause the x-ray detector to prematurely believe that it is satisfied with the overall intensity level of the incident x-rays when, in fact, the intensity level of the x-rays passing thought the patient's anatomy may be below that needed to obtain a high quality image. As a result, the portion of the x-ray detector corresponding to the anatomy of interest is often underexposed to x-rays.
Another difficulty associated with having gaps in the imaging region through which x-rays may pass unattenuated, relates to gray scale mapping which occurs prior to image display on a monitor. More specifically, image quality is substantially based on the ability to see contrast between certain types of anatomy on the monitor. Therefore, the range of energy levels as accumulated by each pixel of the x-ray detector during an exposure is mapped to a gray scale consisting of, for example, 256 steps from black to white. Detected x-rays which are mapped into a central portion of the 256 steps will typically provide sufficient contrast to readily distinguish among different anatomy whereas images mapped near the upper or lower extremities of the gray scale will typically fail to exhibit enough image contrast to properly distinguish and interpret such portions of the image. Unfortunately, x-rays which reach the detector with little to no attenuation through gaps in the patients anatomy will typically cause mapping of the final image to be skewed such that the area of relevant anatomy is mapped to an extreme of the gray scale where there in not enough image contrast to distinguish between relevant anatomy. This occurs since the detector pixels receiving the unattenuated x-rays cause there to be a wide dynamic range of energies to be mapped in most of the gray scale steps (i.e. 256 steps) thereby causing the range of energies holding the relevant image data to be mapped to region having lower overall gray scale contrast.
One known way to reduce the effect of unattenuated x-rays as seen by an x-ray detector is to introduce aluminum bars in the gaps between the patient's anatomy. Such aluminum bars are typically sized and shaped to provide x-ray attenuation similar to the attenuation of an x-ray beam passing through a patient's body. For instance, the aluminum bars may be approximately one inch thick and several inches long. By placing one or more of the aluminum bars in the gaps between the patient anatomy in the imaging region, the number of unattenuated x-rays reaching the x-ray detector is reduced. Unfortunately, the rigidity of the aluminum bars makes it difficult to completely block all unattenuated x-rays from reaching the x-ray detector as the bars do not conform to curves and other shapes which may exist with respect to the patient's anatomy. Thus, the mapping of the x-ray energies received by the x-ray detector to a gray scale will still often result in pertinent anatomy being mapped to a region having insufficient contrast since some unattenuated x-rays often still bombard the x-ray detector thereby skewing the mapping as discussed above.
Another device for reducing the effect of unattenuated x-ray reaching the x-ray detector is to use a compensation filter which is specifically designed for a region of interest to be imaged. Compensation filters are typically a rigid lead-plastic filter which is placed on top of the anatomy of interest and is specially sized and shaped to reduce the large dynamic range of energies which are incident upon the x-ray detector. Compensation filters of this type are commercially available from Nuclear Associates of Carle Place, New York. Unfortunately, because compensation filters are typically designed for imaging of a specific region having generally known attenuation characteristics, such compensation filters are not often suitable when imaging other anatomy which may have varying sized and shaped gaps in the imaging region.
Another difficulty associated with x-ray angiographic imaging techniques is that real time images may become blurred by virtue of movement by a patient during the imaging procedure. To reduce such movement, patients may at times be strapped or otherwise secured to the patient support however such physical constraints are often discomforting to the patient and, depending on the material of the constraint used, could lead to the introduction of artifacts in the resultant image. If patient supports are not used then, of course, there is a greater possibility that the patient may move thereby blurring the final image.
Therefore, what is needed is a method and apparatus which overcomes the shortfalls discussed above and others.